As a wavelength division multiplexing-passive optical network (WDM-PON) capable of providing a high capacity communication service through wavelength allocation becomes important, a development of light sources to be used in the WDM-PON has been significant. The WDM-PON needs a high-speed modulation tunable laser module capable of finely changing wavelengths of respective channels while changing the wavelengths of the channels having a predetermined wavelength interval, and performing high-speed modulation.
A representative high-speed tunable laser module among the high-speed tunable laser modules proposed up to now is a tunable laser module using a sampled grating distributed Bragg reflector (SG-DBR) disclosed in U.S. Pat. No. 4,896,325, and has a laser structure for modulating an optical signal output from the sampled grating distributed Bragg reflector by forming resonance of laser by integrating a gain range and a phase control range between two sampled grating distributed Bragg reflectors and then integrating an optical modulator at a distal end of one sampled grating distributed Bragg reflector between the two sampled grating distributed Bragg reflectors. The sampled grating distributed Bragg reflector has a narrow tunable range of 10 nm or less, and the Vernier effect by the two sampled grating distributed Bragg reflectors is used in order to obtain a wide tunable range.
Further, in order to substitute the tunable laser module using the Bragg grating reflector, a monolithic integrated tunable laser module using two ring resonators having slightly different free spectral ranges (FSR) was published (Thesis: PHOTONICS TECHNOLOGY LETTERS, Vol. 14, No. 5, p 600, 2002, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 21, No. 13, p 851, 2009, and IEEE Journal of Lightwave Technology, Vol. 24, No. 4, p 1865, 2006). In the thesis, an output wavelength of the laser is varied by an interval of the free spectral ranges by fixing a refractive index of one ring resonator and varying a refractive index of the other ring resonator.
Further, a Y-branch tunable laser module in which a laser output terminal is positioned in a gain range, which is different from the tunable laser module using the sampled grating distributed Bragg reflector, was introduced (Thesis: Proceedings Symposium IEEE/LEOS Benelux, p 55, 2003). The Y-branch tunable laser module has a structure in which an optical signal output from one output terminal in the gain range is distributed to two tunable reflectors having different free spectral ranges by using an optical distributor, and outputs a ranged signal to the remaining one output terminal in the gain range. The Y-branch tunable laser module may obtain a wide tunable range by using the Vernier effect identically to the tunable laser module using the sampled grating distributed Bragg reflector. Further, the Y-branch tunable laser module may have a structure in which an additional reflector is integrated in the output terminal of the gain range and then an optical modulator and an optical amplifier are integrated for the high-speed modulation (Thesis: IEE Electronics Letters, vol. 43, no. 9, 2007).
FIG. 1 is a diagram illustrating a structure of a conventional Y-branch tunable laser module.
Referring to FIG. 1, an optical signal output from an one-side cross section of a light gain area unit 101 is separated into two optical signals by an optical distributor 102, the two optical signals are reflected to the light gain area unit 101 by sampled grating distributed Bragg reflectors (SG-DBRs) 104 and 105 having different free spectral ranges again, and the reflected optical signals are reflected in an opposite-side cross-section 106 of the light gain area unit 101 to oscillate a laser. Here, the two reflectors 104 and 105 reflect multi-wavelength optical signals, and the single-wavelength laser is output in one matched wavelength among the reflective wavelengths of the two reflectors 104 and 105 (Vernier effect). Further, since reflection and output need to be generated in the opposite-side cross section 106 of the light gain area unit 101, reflectivity is required to be adjusted by coating the opposite-side cross section 106 of the light gain area unit 101 so that the optical signal is not totally reflected, but partially reflected. That is, an output characteristic is determined according to a degree of the coating of the opposite-side cross section 106 of the light gain area unit 101. Further, one reflector between the two reflectors 104 and 105 includes a phase unit 103, so that phases of the optical signals reflected from the two reflectors 104 and 105 are controlled.
FIG. 2 is a diagram illustrating a structure of a tunable laser module in which a Mach-Zehnder optical modulators is integrated in a conventional Y-branch tunable laser module in order to use the tunable laser module for high-speed modulation and long-term transmission.
Referring to FIG. 2, the light gain area unit 101 does not have a cross-section used for reflection and output, so that a reflection unit 206 is additionally integrated for the laser resonance. Further, in order to manufacture a Mach-Zehnder optical modulator, an optical distributor 204 for separating an optical signal into two optical signals and an optical coupler 201 for combining the two optical signals are used. In addition, the tunable laser module includes phase units 202 and 203 so that the two optical signals have different phases of 180°. Here, the characteristic of the optical modulator is determined by an accurate distribution ratio of the optical distributor 204 used for constituting the Mach-Zehnder optical modulator. Further, an optical amplifier 205 is used in order to amplify the optical signal output from the reflection unit 206.
FIG. 3 is a diagram illustrating a structure of a tunable laser module using a conventional sampled grating distributed Bragg reflector.
Referring to FIG. 3, an optical gain unit 303 and two multi-wavelength reflection units 302 and 304 having different free spectral ranges are used, so as to vary a wide wavelength range by the Vernier effect. An optical amplifier 301 is used in order to amplify a single wavelength laser optical signal output from one of the two multi-wavelength reflection units 302 and 304, and the optical coupler 201 and the optical distributor 204 are used in order to constitute the Mach-Zehnder optical modulator. Further, the tunable laser module includes the phase units 202 and 203 so that the two optical signals separated by the optical distributor 204 have different phases of 180°.
However, the Mach-Zehnder optical modulator formed of the optical coupler, the optical distributor and two arms needs to be separately manufactured in the conventional tunable laser module, so that a structure of the tunable laser module is complex and it is difficult to achieve the monolithic integration.